Post-Polio Syndrome (cont.)

The cause is unknown. However, the new weakness of post-polio syndrome appears to be related to the degeneration of individual nerve terminals in the motor units that remain after the initial illness. A motor unit is a nerve cell (or neuron) and the muscle fibers it activates. The poliovirus attacks specific neurons in the brainstem and the anterior horn cells of the spinal cord. In an effort to compensate for the loss of these neurons, ones that survive sprout new nerve terminals to the orphaned muscle fibers. The result is some recovery of movement and enlarged motor units.

Years of high use of these enlarged motor units adds stress to the neuronal cell body, which then may not be able to maintain the metabolic demands of all the new sprouts, resulting in the slow deterioration of motor units. Restoration of nerve function may occur in some fibers a second time, but eventually nerve terminals malfunction and permanent weakness occurs. This hypothesis is consistent with post-polio syndrome's slow, stepwise, unpredictable course.

Through years of studies, scientists at the National Institute of Neurological Disorders and Stroke and at other institutions have shown that the weakness of post-polio syndrome is a very slowly progressing condition marked by periods of stability followed by new declines in the ability to carry out usual daily activities.

Source: MedicineNet.com
http://www.medicinenet.com/post-polio_syndrome/article.htm